Green’s Dictionary of Slang

1. a weapon.

1904		Lancs. Eve. Post 20 Oct. 5/4: Soft End o’t Poker. Eawr [our] Bet’s Infallible Convincer.
1904		Sydney Sportsman (Surry Hills, NSW) 10 Aug. 6/2: [H]is punch is an almost certain convincer if it lands right.
1935		A.J. Pollock Und. Speaks.
1942		N. Algren Never Come Morning (1988) 20: Take the crank before we go in [...] Use it fer a jimmy first ’n fer the convincer later.
1950		Goldin et al. DAUL 48/2: Convincer, n. [...] 2. A pistol, knife, blackjack, or anything that convinces the victim of his personal jeopardy.

2. a persuasive action or speech.

1908		B. Fisher A. Mutt in Blackbeard Compilation (1977) 154: Our hero was about to toss in the sponge, but Cicero was there with the convincer.
1938		D. Runyon ‘It Comes Up Mud’ in Runyon on Broadway (1954) 531: The only diamond she ever sees is an engagement hoop that Little Alfie gives her as the old convincer.
1941		B. Schulberg What Makes Sammy Run? (1992) 165: That, as Sammy would say, was the convincer.
1953		A.J. Liebling Honest Rainmaker (1991) 104: The ‘disappointment’ about winning only $540 was the convincer. Readers could see Maxim & Gay were on the level.

3. (US Und.) an extra sum of money committed by the victim of a confidence trick that ultimately locks him into the scheme.

1913

Sun (NY) 27 July 40/2: The booster tells Boob that he’ll have to get the convincer. Maybe he can get it in some New York bank [...] It’s not easy and the booster has to rig up a fake explanation to fool the cashiers.

4. (US Und.) luring a victim into a confidence trick by allowing them to make an initial profit.

1910		B. Fisher Mutt & Jeff 4 Oct. [synd. strip] We’ll let him him win a couple of dollars first as a convincer, then we’ll cold deck him and cop his roll.
1940		D. Maurer Big Con 4: Allowing the victim to make a substantial profit. (Giving him the convincer.).